Abrading machines

ABSTRACT

This application discloses a machine for abrading a workpiece with a blast of abrasive in which the reusable abrasive and detritus are removed by suction from the workpiece and the reusable abrasive is returned for redirection at the workpiece through the airlock arrangement of three hoppers, the middle one of which has its pressure changed periodically. The pressure changes in the middle hopper are effected automatically and cyclically by a diaphragm connected to a source of suction so that the suction forces gradually build up on one face of the diaphragm while the pressure in the middle hopper is at one value, then the diaphragm moves in response to said forces and thus changes the pressure in the hopper to another value and then the diaphragm returns to its initial position and the suction forces gradually build up again to commence another cycle.

United States Patent Field, deceased et al.

[ 51 Feb. 8, 1972 [54] ABRADING MACHINES Abrasive Development Limited, Henley in Arden, Solihull, Warwickshire, England [22] Filed: Aug. 19, 1969 [21] Appl.No.: 852,155

[73] Assignee:

Primary Examiner-Lester M. Swingle Attorney-Kurt Kelman [57] ABSTRACT This application discloses a machine for abrading a workpiece with a blast of abrasive in which the reusable abrasive and detritus are removed by suction from the workpiece and the reusable abrasive is returned for redirection at the workpiece through the airlock arrangement of three hoppers, the middle one of which has its pressure changed periodically. The pressure changes in the middle hopper are effected automatically and cyclically by a diaphragm connected to a source of suction so that the suction forces gradually build up on one face of the diaphragm while the pressure in the middle hopper is at one value, then the diaphragm moves in response to said forces and thus changes the pressure in the hopper to another value and then the diaphragm returns to its initial position and the suction forces gradually build up again to commence another cycle.

7 Claims, 5 Drawing Figures PATENTED FEB 8 I972 SHEEI 1 OF 5 PATENTED FEB. 8 I972 SHEET t (If 5 ABRADING MACHINES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to abrading machines operating on dry blast in which a stream of air and abrasive is directed against a workpiece, abrasive and detritus are removed from the workpiece and reusable abrasive is separated from the detritus for redirection at the workpiece.

2. Description of Prior Art In U.S. Pat. No. 3,307,296, issued 7th Mar. 1967 to Abrasive Developments Limited there is described an abrading machine comprising directing means for directing air and abrasive against a workpiece, an input pipe for conveying air and abrasive to the directing means, a suction pipe for conveying abrasive and detritus from adjacent the workpiece, a separator connected to the suction pipe to separate reusable abrasive from the material delivered by the suction pipe, an upper hopper fed with reusable abrasive from the separator and in communication with the suction pipe, a lower hopper arranged to deliver abrasive by gravity into the input pipe, means maintaining the interior of the lower hopper at substantially the same pressure as that in the input pipe, a middle hopper interposed between the upper and lower hoppers and communicating with them both, first valve means for selectively placing the interior of the middle hopper at the pressure either of the suction pipe or the input pipe, and abrasive controlling second and third valve means between the upper and middle hoppers and the middle and lower hoppers respectively, each of said second and third valve means comprising a valve member freely movable between open and closed positions in response to the net resultant force exerted on the member by differences in the pressures in the hoppers between which the valve member is interposed and the downward force exerted on the member by any abrasive in the hopper above thevalve member, the arrangement being such that when the middle hopper is at the input pipe pressure, the second valve means is closed and the third valve means is open and, when the middle hopper is at the suction pipe pressure, the second valve means is open and the third valve means is closed.

In the above arrangement the first valve means included a valve and cycling means so that the valve is continually changed over. It is an object of the present invention to provide an improvement in the abrading machine described above and particularly in relation to the first valve means.

SUMMARY OF THE INVENTION The invention is concerned with abrading machines, hereinafter referred to as being of the type specified, comprising directing means for directing air and abrasive against a workpiece, input conduit means for conveying air and abrasive to the directing means, suction conduit means for conveying abrasive and detritus from adjacent the workpiece, a separator connected to the suction conduit means to separate reusable abrasive from the abrasive detritus delivered by the suction conduit means, an upper hopper fed with reusable abrasive from the separator and in communication with the suction conduit means, a lower hopper arranged to deliver abrasive for reuse, a middle hopper interposed between the upper and lower hoppers and communicating with them both, first valve means for selectively placing the interior of the middle hopper at the pressure either of the upper hopper or of the lower hopper, and abrasive controlling second and third valve means between the upper and middle hoppers and the middle and lower hoppers respectively, each of said second and third valve means comprising a valve member freely movable between open and ciosed positions in response to the net resultant force exerted on the member by differences in the pressures in the hoppers between which the valve member is interposed and the downward force exerted on the member by any abrasive in the hopper above the valve member, the arrangement being such that when the middle hopper is at the pressure of the lower hopper the second valve means is closed and the third valve means is open and, when the middle hopper is at the pressure of the upper hopper, the second valve means is open and the third valve means is closed.

According to the invention we provide an abrading machine of the type specified wherein the first valve means comprises a diaphragm arranged cyclically to control the pressure in the middle hopper and having a first position in which the pressure in the middle hopper is the pressure in the upper hopper and a second position in which the pressure in the middle hopper is the pressure in the lower hopper, the diaphragm being constantly urged to one of said positions, and means connecting one face of the diaphragm to a source of suction in such a manner that when the machine is in operation the suction forces gradually build up on said one face and, when they exceed a predetermined value, move the diaphragm to the other of said positions, movement of the diaphragm to the other of said positions releasing the suction on said one face so that the diaphragm returns to said one position to allow the commencement of another cycle.

Assuming that the diaphragm is urged to its first position, the pressure in the middle hopper will be the pressure in the upper hopper and the second valve means can open so that abrasive passes from the upper hopper into the middle hopper. As the suction on the one face of the diaphragm increases, there will come a point when it will overcome the forces urging the diaphragm to its first position and the diaphragm will then move to its second position in which the pressure in the middle hopper is the pressure in the lower hopper and as a result the second valve means will close and the third valve means will open thus allowing abrasive to move from the middle hopper to the lower hopper. Upon the release of the suction, the diaphragm will again move to its first position thus placing the middle hopper at the pressure of the upper hopper thus closing the third valve means and opening the second valve means and allowing abrasive to pass from the upper hopper to the middle hopper. This cycling will continue and will depend, for its timing, on the time taken to build up the suction forces on the one face of the diaphragm to a value such as to overcome the forces urging the diaphragm to its one position.

In a preferred arrangement, both faces of the diaphragm are connected, when the machine is in operation, to said source of suction but the area of said one face which is connected to the source of suction is greater than the area of the other face which is connected to the source of suction. As a result, as the suction forces build up on the one face they will also build up on the other face, the forces on the other face tending to hold the diaphragm in its one position until, when the forces on the one face reach a predetermined value, the diaphragm will move over to the other of its positions.

In a preferred arrangement there is a first pipe which is connected to the middle hopper and which has its end closed by said other face of the diaphragm when the latter is in said one position, the pipe being opened to the pressure in the lower hopper when the diaphragm is in the other of said positions, the pipe being connected through a restriction to the suction means and, through a second pipe, to said one face of the diaphragm. In this arrangement, therefore, both faces of the diaphragm are exposed to suction from the same source.

The second pipe may include a biasing valve so arranged as to control the rate of the buildup of the suction forces on the one face of the diaphragm when the latter is in said one position but so as to give clear passage to air flowing to said one face when the diaphragm moves to the other of said positions.

A one-way valve may be provided in the second pipe to allow air to flow to said one face of the diaphragm when the latter has moved to the other of its positions but to prevent airflow to said face when the diaphragm is in said one position.

Preferably, the diaphragm is made of resilient material and is urged to said one position by its natural resilience and, when said other face is also exposed to the source of suction, the suction forces help to hold the diaphragm closed until the forces on the, one face overcome both the forces of natural resilience and the suction forces.

Normally, the lower hopper pressure will be atmospheric.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in detail by way of example with reference to the accompanying drawings in which:

FIG. 1 is an elevational view of the main part of an abrading machine embodying the invention with certain parts omitted;

FIG. 2 is a diagrammatic section through an abrading head which can be used with this invention;

FIG. 3 is a section showing the means for distributing abrasive from the lower hopper;

FIG. 4 is a cross section through the diaphragm mounting showing the diaphragm in its one position; and

FIG. 5 is a cross section through the biasing valve means to control the rate at which the diaphragm moves back to its one position after having assumed its other position.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIGS. 1 and 2 the abrading machine comprises four parts, the abrading device shown in FIG. 2, the means for separating reusable abrasive indicated generally at 11 in FIG. 1, dust-collecting means indicated generally at 12 in FIG. 1 and suction means indicated generally at 13 in that figure.

Dealing first with the abrading device shown in FIG. 2, this is shown as being applicable to the cleaning of one surface of a sheet 14 but the design of the abrasive head is not part of the present invention and any convenient form of head may be used. In the example shown, the head comprises a chamber 15 having a generally cylindrical portion 16 and winglike portions 17 and 18. Thesheet 14 is passed through guides 19 and 20 in the winglike portions 17 and 18. Suction is applied to the chamber via a suction conduit 21 which is connected to the suction 'means 13 in a manner which will hereinafter be described, Suction applied to the chamber 15 removes air therefrom and thus causes a mixture of abrasive and primary air to be drawn into the chamber along the hose 22 to the nozzle 23. The guides 19 and 20 restrict the entry of secondary air into the chamber 15 as the sheet 14 passes through the chamber and the suction in the chamber is sufficient therefore to impel primary air and abrasive into the chamber. ,lmmediately before entering the chamber tertiary air is admitted to the mixture of primary air and abrasive entering the nozzle through a number of apertures one of which is shown at 24. There are a number of such nozzles 23 arranged in a line perpendicular tothe plane of FIG. 2 and each is supplied with primary air and abrasive through a separate hose 22 by means which will hereinafter be described. The abrasive leaving the nozzle 23 is caused to impinge on the surface 25 of the sheet 14 and the abrasive is then removed from the chamber 15 with the air through the suction conduit 21.

The separating means 11 comprises a cyclone 26 to which the suction conduit 21 is connected at 27. The cyclone 26 separates reusable abrasive from the mixture of air and abrasive which it receives from the suction conduit 21, the reusable abrasive falling to the bottom of the cyclone as will hereinafter be described and the dust and air passing through a pipe 270 into a second cyclone 28 forming part of the dustcollecting means. The dust falls to the bottom of the cyclone and is received in a dust box 29 from which it may be removed. The air leaving the cyclone 28 passes through pipework' indicated generally at 30 to the bottom of a bag filter 31 where any residual dust is separated out and received in a dust box 32, the substantially dust-free air then passing through the pipework 33 through an exhauster fan 34 driven by an electric motor 35. It will be seen, therefore, that the exhauster fan 34 applies suction to the suction conduit 21 via the cyclones 26 and 28 and the filter 31 and thus is responsible for drawing primary air and abrasive into the chamber 15.

The reusable abrasive which falls to the bottom of the cyclone 26 is deflected by a baffle 36 and is received in an upper hopper 37. From the upper hopper 37 the abrasive can pass into a middle hopper 38 when the pressures in the two hoppers are equal. The pressure in the hopper 38 is varied by means of first valve means 39 hereinafter to be described in detail. Second valve means is interposed between the upper hopper 37 and the middle hopper 38, is indicated generally at 40 and comprises a flap 41 of hard rubber or similar material freely pivoted about its upper edge so as to be movable in response to the net resultant force exerted on it by the difference in the pressures between the hoppers 37 and 38 and any abrasive in the hopper 37 tending to open the flap 41. The middle hopper 38 is sealed at its top at 42 and is in airtight relation with the upper hopper 37.

An open topped, generally cylindrical lower hopper 43 is provided having within its base a conical baffle 44. Third valve means indicated generally at 45 is interposed between the middle hopper 38 and the lower hopper 43, the third valve means comprising a flap 46 mounted in a manner similar to the flap 41 Le, pivoted about its upper edge so that it will move in response to the net resultant force exerted on it by differences in the pressures between the hoppers 38 and 43 and the weight of any abrasive in the hopper 38 acting on the flap 46.

As will be described in detail below, the first valve means 39 operates so that the middle hopper 38 is alternately placed at the pressure of the upper hopper 37 and at the pressure of the lower hopper 43 i.e., atmospheric pressure. When the middle hopper 38 is at the pressure of the upper hopper 37, atmospheric pressure closes the third valve means 45 but the second valve means 40 can open to let abrasive pass from the upper hopper 37 to the middle hopper 38. When the middle hopper 38 is placed at atmospheric pressure i.e., the pressure of the lower hopper 43, the second valve means 42 is maintained'closed by the atmospheric pressure acting on the flap 41 while the third valve means 45 can open to transfer abrasive from the middle hopper 38 to the lower hopper 43.

Abrasive delivered into the lower hopper 43 is guided by the baffle 44 to the lower peripheral portion of the hopper and from thence is entrained in input conduits formed by the hoses such as 22 referred to in FIG. 2.

Referring to FIG. 3, the base of the lower hopper is formed by a ring 47 to which is welded the lower edges of the baffle 44 and also the lower edge of the outer wall 48 of the hopper. In the ring 47 there is provided a plurality of circular threaded apertures 49 in each of which is threadedly received a nozzle member 50 through which the abrasive can pass at a metered rate. Arranged beneath the ring 47 is a further ring 51 which has a plurality of cylindrical apertures 52 therein, the apertures 52 being coaxial with the apertures 49. Welded to the upper surface of the ring 51 coaxial with each aperture 52 is a sleeve 53 into which the nozzle member 50 directs the abrasive. The base of each aperture 52 is closed by a plug 54. The ring 51 also has a plurality of transverse apertures, one of which is indicated at 55, each transverse aperture intersecting an aperture 52 and threadedly receiving a union member 56 to which is clamped one end of a hose 22 which forms an inlet conduit, the other end of the hose 22 being connected to a nozzle 23 in the chamber 15. It will be appreciated that each of the nozzles 23 is supplied with air and abrasive along its own hose or inlet conduit 22.

When suctionis applied to the chamber 15, primary air is introduced into the upper end of each sleeve 53 and draws abrasive out of the lower hopper 43 through the nozzle members 50. The mixture of primary air and abrasive then passes along the hoses or input conduits 22 to the nozzles 23 where the mixture is directed at the surface 25 of the sheet 14 and the abrasive and air is then returned along the suction conduit 21 as has been described above.

The present invention is concerned mainly with the first valve means 39 and these will now be described in detail with reference to FIGS. 1, 4 and 5.

The first valve means comprises a housing indicated generally at 57 and comprising first and second housing parts 58 and 59 having flanges 60 and 61 respectively which are bolted together by bolt assemblies 62. Sandwiched between the flanges 60 and 61 is the edge portion of a diaphragm indicated generally at 63, the edge portion thereof being tightly clamped between the flanges 60 and 61 in airtight relation therewith. The diaphragm 63 is made from natural or synthetic rubber and is resilient and the edges of the housing parts 58 and 59 between which it passes are rounded to prevent damage thereto.

The housing is mounted on a first pipe indicated generally at 64 and comprising two pipe parts 65 and 66 which have flanges 67 and 68 respectively which are bolted together by bolts (not shown) with the interposition of a washer 69. The pipe part 65 is connected to the middle hopper 38 and communicates with the interior thereof. The pipe part 64 has welded thereto the second housing part 59 and has a rounded end 70 with which one face 71 of the diaphragm 63 engages when the diaphragm is in the position shown in FIG. 4. It will be seen that a portion 72 of the face 71 is exposed to the pressure in the first pipe 64. The remaining portion 73 of the face 71 is open to atmosphere through a plurality of holes 74 formed in the wall of the second housing part 59. The first pipe 64 is connected via a pipe 75, shown in FIG. 1, with the suction side of the cyclone 26 i.e., with the low pressure in the suction conduit 21. The pipe 75 contains a restriction 76.

The housing part 58 of the housing 57 is connected by a second pipe 77 to the pipe part 65 as shown in FIG. 1. This pipe 77 contains a filter 78 and a valve assembly 79 which is shown in detail in FIG. 5 to which reference will now be made.

The valve assembly comprises a biasing ball valve indicated generally at 80 and a one-way ball valve indicated generally at 81. The biasing valve 80 comprises a ball 82 received in a tubular housing 83 whose ends are closed by union members 84 and 85. The union member 84 has one part of pipe 77 connected thereto which extends to the filter 78. The right-hand end of the union member 85 is provided with a conical seat 86 having diametrically opposed slots 87 therein. The housing 83 is also provided with a transverse stop pin 88 to limit movement of the ball 82 to the right in FIG. 5.

The union member 85 is engaged in one limb of a T-shaped housing and is connected to another part of the pipe 77. The transverse limb 90 of the housing is engaged by union member 91 which at its lower end is provided with a conical seating 92 against which can seat a ball 93. Also engaged with the union member 91 is a cylindrical housing 94 within which the ball valve 81 is housed, the housing having a stop pin 95 extending transversely thereof to prevent the ball 93 of ball valve 81 falling out.

The balls 82 and 93 should so engage their seatings 86 and 92 that they engage the flanks thereof rather than the orifices since otherwise the balls will be liable to stick during operation of the apparatus which will now be described, it being assumed that the lower hopper 43 already contains some abrasive and that the diaphragm 63 is in the position shown in FIG. 4 which is the position which it will normally assume under the effect of its own natural resilience, that is to say the end 70 of the first pipe 64 will be closed by the diaphragm.

If now the suction means 13 is operated by starting the motor 35 thus driving the exhauster fan 34, the bag filter 31 and the cyclones 26 and 28 will be placed under suction. The suction conduit 21 will be placed under suction and will commence to draw air out of the chamber 15. As a result of drawing air out of the chamber, primary air will be caused to flow into the sleeves 53 on the ring 51 below the lower hopper 43 and abrasive will be drawn out of the lower hopper and the mixture of primary air and abrasive will travel along the hoses or input conduits 22 to the nozzles 23. Tertiary air will be entrained through the apertures 24 and the resulting mixture of air and abrasive will be directed at the surface of the sheet 14 which will be moved through the chamber in the direction of the arrow 96. Abrasive and air will then pass along the suction conduit 21 into the cyclones 26 and 28. Reusable abrasive will be separated out and will fall to the bottom of the cyclone 26 and the dust and spent abrasive will be extracted from the cyclone 28 and the filter 31 as described above.

As suction is built up in the cyclone 26, the pressure will drop in the first pipe 64 due to the connection of the pipe to the suction pressure through the pipe 75 and the restriction 76, the middle hopper 38 will thus be at the pressure of the suction conduit and thus the first valve means 40 will be free to open under the weight of any abrasive in the cyclone and the third valve means 45 will be held closed by the effect of atmospheric pressure on the flap 46.

Air will also be removed from the volume 97 formed in the first housing part 58 by the diaphragm 63. This air will be removed through the pipe 77 which is connected to the volume 97 and also to the first pipe 64. The low pressure in the pipe 64 will force the balls 82 and 93 in the valve assembly 79 onto their seatings 86 and 92 respectively but due to the slots 87 in the seating 86, air will be gradually removed from the volume 97, the rate of removal depending on the size of the slots 87.

Considering now the forces acting on the diaphragm 63, there will be atmospheric pressure acting on the portion 73 of the face 71 of the diaphragm tending to move the diaphragm to the right in FIG. 4. This movement will be resisted by the pressure in the volume 97 and the pressure difference between the pressure in the volume 97 and the pressure in the first pipe 64 acting over the area 72 of the face 71. As the pressure in the volume 97 decreases, however, due to the removal of air therefrom, there will come a time when the pressure in the volume 97 is insufficient to resist the force acting on the portion 73 of the face 71 of the diaphragm and the diaphragm will move to the right with a snap action. As a result, the pipe 64 will be opened to atmosphere through the housing part 59 and the holes 74. The pressure in the middle hopper 38 will thus become atmospheric thus acting on the flap 41 to close the second valve means 42 and allowing the third valve means 45 to open to deposit any abrasive in the middle hopper into the lower hopper.

Upon the opening of the pipe 64 to atmospheric pressure, the balls 82 and 93 will no longer be forced onto their seats and the ball 93 will drop against its stop pin to allow air to enter the volume 97 thus to return the diaphragms 63 to the position shown in FIG. 4. The provision of the valve 81 allows the air to enter the volume 97 rapidly after the diaphragm 63 has been moved to the right in FIG. 4 so as to ensure that it snaps back into position and does not move slowly back into position as it might if the valve 81 were not provided since the moving of the diaphragm back into position without the provision of the valve 81 could result in the formation of a vacuum in the volume 97. The ball 93 will be pulled back onto its seating either towards the end of the inflow of air into the volume 97 or at the latest when the pressure decreases again in the first pipe 64.

It will be appreciated that once the diaphragm 63 has returned to the position shown in FIG. 4, air will be removed from the middle hopper 38 through the pipe 75 so that the middle hopper 38 will again assume the pressure of the suction conduit to allow the second valve means 42 to open and closing the third valve means 45 and also air will gradually be removed from the volume 97 until the diaphragm 63 again moves to the right to open the middle hopper 38 to atmospheric pressure.

It will be seen that the first valve means 39, therefore, acts so as cyclically to place the middle hopper 38 at the pressure of the lower hopper, in this case atmospheric pressure, and the pressure of the upper hopper so that the second and third valve means 40 and 45 alternately open and close thus allowing abrasive to be transferred from the upper hopper to the lower hopper without unduly disturbing the low pressure in the system.

Although the invention has been described in detail with reference to a particular form of abrading device, i.e., that shown in FIG. 2 which forms no part of the present invention any convenient form of abrading device may be used and the invention is not limited to an arrangement in which the primary air and abrasive is introduced into the abrading device solely by suction as described.

It is claimed:

1. An abrading machine comprising directing means for directing air and abrasive against a workpiece; input conduit means for conveying air and abrasive to the directing means; suction conduit means for conveying abrasive and detritus from adjacent the workpiece; a separator connected to the suction conduit means to separate reusable abrasive from the abrasive and detritus delivered by the suction conduit means; an upper hopper fed with reusable abrasive from the separator and in communication with the suction conduit means; a lower hopper arranged to deliver abrasive for reuse, a middle hopper interposed between the upper and lower hoppers and communicating with them both; first valve means for selectively placing the interior of the middle hopper at the pressure either of the upper hopper or of the lower hopper, the first valve means comprising a diaphragm arranged cyclically to control the pressure in the middle. hopper and having a first position in which the pressure in the middle hopper is the pressure in the upper hopper and a second position in which the pressure in the middle hopper is the pressure in the lower hopper, the diaphragm being constantly urged to one of said positions, and means connecting one face of the diaphragm to a source of suction in such a manner that when the machine is in operation the suction forces gradually build up on said one face and, when they exceed a predetermined value, move the diaphragm to the other of said positions, movement of the diaphragm to the other of said positions releasing the suction on said one face so that the diaphragm returns to said one position to-allow the commencement of another cycle; and abrasive-controlling second and third valve means between the upper and middle hoppers and the middle and lower hoppers respectively, each of said second and third valve means comprising a valve member freely movable between open and closed positions in response to the net resultant force exerted on the member by differences in the pressures in the hoppers between which the valve member is interposed and the downward force exerted on the member by any abrasive in the hopper above the valve member, the second valve means being closed and the third valve means being open when the middle hopper is at the pressure of the lower hopper and the second valve means being open and the third valve means being closed when the middle hopper is at the pressure of the upper hopper.

2. A machine according to claim 1 wherein both faces of the diaphragm are connected, when the machine is in operation. to said source of suction, the area of said one face which is connected to the source of suction being greater than the area of the other face which is connected to the source of suction.

3. A machine according to claim 2 including a first pipe which is connected to the middle hopper at one of its ends and which has the other of its ends closed by the other face of the diaphragm when the latter is in said one position, said other end of the pipe being opened to the pressure in the lower hopper when the diaphragm is in the other of said positions, the pipe being connected through a restriction to the source of suction and through a second pipe to said one face of the diaphragm.

4. A machine according to claim 3 wherein the second pipe includes a biasing valve arranged to control the rate of buildup of the suction forces on the one face of the diaphragm when the latter is in said one position but so as to give clear passage to air flowing to said one face when the diaphragm moves to the other of said positions.

5. A machine according to claim 3 wherein the second pipe includes a one-way valve to allow air to flow to said one face of the diaphragm when the latter has moved to the other of its positions but to prevent airflow to said face when the diagahragm is in said one position.

A machine according to claim 1 wherein the diaphragm is made of resilient material and is urged so said one position by its natural resilience.

7. A machine according to claim 1 wherein the lower hopper is open to atmospheric pressure. 

1. An abrading machine comprising directing means for directing air and abrasive against a workpiece; input conduit means for conveying air and abrasive to the directing means; suction conduit means for conveying abrasive and detritus from adjacent the workpiece; a separator connected to the suction conduit means to separate reusable abrasive from the abrasive and detritus delivered by the suction conduit means; an upper hopper fed with reusable abrasive from the separator and in communication with the suction conduit means; a lower hopper arranged to deliver abrasive for reuse, a middle hopper interposed between the upper and lower hoppers and communicating with them both; first valve means for selectively placing the interior of the middle hopper at the pressure either of the upper hopper or of the lower hopper, the first valve means comprising a diaphragm arranged cyclically to control the pressure in the middle hopper and having a first position in which the pressure in the middle hopper is the pressure in the upper hopper and a second position in which the pressure in the middle hopper is the pressure in the lower hopper, the diaphragm being coNstantly urged to one of said positions, and means connecting one face of the diaphragm to a source of suction in such a manner that when the machine is in operation the suction forces gradually build up on said one face and, when they exceed a predetermined value, move the diaphragm to the other of said positions, movement of the diaphragm to the other of said positions releasing the suction on said one face so that the diaphragm returns to said one position to allow the commencement of another cycle; and abrasive-controlling second and third valve means between the upper and middle hoppers and the middle and lower hoppers respectively, each of said second and third valve means comprising a valve member freely movable between open and closed positions in response to the net resultant force exerted on the member by differences in the pressures in the hoppers between which the valve member is interposed and the downward force exerted on the member by any abrasive in the hopper above the valve member, the second valve means being closed and the third valve means being open when the middle hopper is at the pressure of the lower hopper and the second valve means being open and the third valve means being closed when the middle hopper is at the pressure of the upper hopper.
 2. A machine according to claim 1 wherein both faces of the diaphragm are connected, when the machine is in operation, to said source of suction, the area of said one face which is connected to the source of suction being greater than the area of the other face which is connected to the source of suction.
 3. A machine according to claim 2 including a first pipe which is connected to the middle hopper at one of its ends and which has the other of its ends closed by the other face of the diaphragm when the latter is in said one position, said other end of the pipe being opened to the pressure in the lower hopper when the diaphragm is in the other of said positions, the pipe being connected through a restriction to the source of suction and through a second pipe to said one face of the diaphragm.
 4. A machine according to claim 3 wherein the second pipe includes a biasing valve arranged to control the rate of buildup of the suction forces on the one face of the diaphragm when the latter is in said one position but so as to give clear passage to air flowing to said one face when the diaphragm moves to the other of said positions.
 5. A machine according to claim 3 wherein the second pipe includes a one-way valve to allow air to flow to said one face of the diaphragm when the latter has moved to the other of its positions but to prevent airflow to said face when the diaphragm is in said one position.
 6. A machine according to claim 1 wherein the diaphragm is made of resilient material and is urged so said one position by its natural resilience.
 7. A machine according to claim 1 wherein the lower hopper is open to atmospheric pressure. 